The visual system of cats exhibits a certain degree of plasticity during its development. The phenomenon has been studied extensively physiologically, but its anatomical bases are only beginning to be understood. In the visual cortex of monocularly deprived (MD) cats, few cells respond to input from the deprived eye, while cells in the lateral geniculate nucleus (lgn) respond relatively normally. This suggests that the connections to visual cortex from the deprived geniculate laminae may have been disrupted. Experiment I will examine these connections in MD cats using both light and electron microscope autoradiography of visual cortex after single-lamina injections of tritiated lysine in lgn. The placement of isotope will be guided by recording cellular activity with the same micropipette used for injection. In the visual cortex of cats exposed only to horizontal or vertical lines during development, most cells respond only to lines of the same orientation as those viewed during development. Experiment II will look for the anatomical basis of this physiological change. The orientation of a cell's receptive field has been postulated to be determined by the orientation of its dendritic field. We will use the Golgi technique to examine cortical neurons selectively-reared cats to see whether there is a shift in the distribution of dendritic field orientations toward the orientation viewed during development. The cats in this experiment are reared viewing one orientation with both eyes. To determine whether changes in dendritic morphology can occur independently within different ocular dominance columns, Experiment III will examine by Golgi and autoradiography the cortex of cats reared viewing horizontal lines with one eye and vertical lines with the other.